1. Field of the Invention
The invention relates to an actuating device for a friction clutch arranged in the drivetrain of an automobile, in particular a utility automobile, between an internal combustion engine and a gear of a casing bell, said actuating device comprising a disengagement bearing arrangement for actuating the friction clutch, said disengagement bearing arrangement being movable essentially coaxially to the friction clutch, and a positioning servo arrangement with a pressure-medium power cylinder arrangement which acts on the disengagement bearing arrangement and can be actuated via a control valve, connected to a pressure-medium source, as a function of a command variable representing a desired position and of an actual value representing the axial position of the disengagement bearing arrangement.
2. Discussion of the Prior Art
An actuating device of this type is known, for example, from DE 33 21 578 C2. The known actuating device has a vacuum booster as a positioning servo arrangement. The booster constructed in the manner of a vacuum brake booster is designed integrally with a pneumatic power cylinder and a control valve and is arranged outside the casing bell. Two working chambers of the pneumatic power cylinder are separated from one another by an axially movably guided piston and an elastic diaphragm. One working chamber, designed as a vacuum chamber, is connected to an intake line system of the internal combustion engine. The other working chamber, serving as a control chamber, can be connected by means of the control valve either to the vacuum chamber or, via a pressure equalizing orifice, to the atmosphere. The vacuum booster is actuated via a control rod which butts against a valve body of the control valve and which can be displaced axially via an electromotively driven cam. Axial displacement of the control rod results in the control valve being switched in such a way that the piston follows the movement of the control rod with power assistance. The movement of the piston acts via a hydraulic master cylinder on a slave cylinder which is connected to the master cylinder and is arranged outside the casing bell and which, in turn, acts on a disengagement fork assigned to the disengagement bearing arrangement. Emergency actuation of the clutch when the internal combustion engine is stationary, that is to say in the absence of a vacuum, is possible due to the fact that the control rod can act directly on the piston via the valve body of the control valve.
The control valve has a valve body which cooperates with an elastic valve seat. Provided in the valve seat is a connecting duct, via which the connection is made between the control chamber and the ambient air, insofar as the valve body is not pressed against the elastic valve seat in order to close the connecting duct by means of the valve body. A further connecting duct connects the control chamber to the vacuum chamber.
Furthermore, an actuating device of the type initially mentioned, which has a pneumatic power cylinder as a pressure-medium power cylinder arrangement, is known. The actuating device is mounted, as an integral unit comprising the pneumatic power cylinder, a hydraulic slave cylinder and the control valve, externally on the casing bell. The piston of the pneumatic power cylinder is mounted on a rod element forming the piston of the hydraulic slave cylinder, and the rod element is connected to a tappet which reaches into the interior of the casing bell and which engages on a disengagement fork assigned to the disengagement bearing arrangement. A master cylinder actuable by the clutch pedal and a control inlet of the control valve are connected to the hydraulic slave cylinder. The control valve controls the supply of compressed air into the pneumatic power cylinder and the bleeding of air from the pneumatic power cylinder as a function of the hydraulic pressure prevailing at the control inlet, in such a way that a predetermined hydraulic pressure, defined by a compression spring arrangement, is established at the control inlet. In this case, the hydraulic slave cylinder serves as a measuring cylinder which records the position of the rod element and, consequently, indirectly the position of the disengagement bearing arrangement. When the master cylinder is actuated, forces are exerted directly on the rod element, and consequently on the disengagement bearing arrangement, via the hydraulic slave cylinder serving as a measuring cylinder, in addition to the actuating forces of the pneumatic power cylinder due to the supply of compressed air into the latter.
The intention is to put an actuating device of the initially specified type on the market, in which the pressure-medium power cylinder arrangement is arranged inside the casing bell. One possible variant is distinguished in that the pressure-medium power cylinder arrangement is designed to exert on the disengagement bearing arrangement a force which is essentially coaxial to the clutch axis. For this purpose, it is considered, for example, that the pressure-medium power cylinder arrangement should comprise an annular pressure-medium cylinder essentially concentric to the clutch axis.
In an arrangement of this type, it will be possible, in principle, to use a control valve of the type described above, with a hydraulic slave cylinder which serves as a measuring cylinder and by which forces are exerted on the disengagement bearing arrangement when the control valve is actuated. A disadvantage of this, however, would be that, in the case of a pressure-medium power cylinder arrangement designed to exert on the disengagement bearing arrangement a force which is essentially coaxial to the clutch axis, tilting moments are exerted on the disengagement bearing arrangement by the measuring cylinder, insofar as the latter is not also designed for exerting on the disengagement bearing arrangement forces which are essentially coaxial to the clutch axis, for example as an annular measuring cylinder essentially concentric to the clutch axis. An annular measuring cylinder of this type would have the disadvantage that, if the hydraulic measuring cylinder has leaks, an exchange of the measuring cylinder would make it necessary to release the casing bell from the internal combustion engine. If, on the other hand, a measuring cylinder arranged, for example, laterally to the pressure-medium power cylinder arrangement were used, the tilting moments then exerted on the disengagement bearing arrangement would require a correspondingly higher outlay in mechanical terms in order to absorb or compensate these tilting moments.